DNS of Turbulent Premixed Ammonia/Hydrogen Flames: The Impact of Thermo-Diffusive Effects

Direct Numerical Simulations (DNS) of three-dimensional premixed turbulent hydrogen-air flames enriched with 19%, 36%, 44% and 57% of NH3 (in volume) are performed. Starting from an equivalence ratio of 0.44 for the case with 19% of NH3, richer mixtures of phi = 0.54, 0.69 and 0.95 are considered when increasing NH3 concentration to obtain comparable laminar flame speeds, i.e., 0.17 m/s for 19% and 36 % NH3 enriched case, and 0.30 m/s when NH3 concentration is increased to 44 and 57%. The composition and characteristics of the studied mixtures enable to investigate the effects of thermo-diffusivity in a turbulent flow and the role of chemistry and stretch effects in the development of the flames. Given a composition of ammonia and hydrogen and an equivalence ratio, a predictive method is described to identify compositions where thermo-diffusive effects impact the flame and predict the stretch factors. Two maps are proposed to achieve this: the first one is based on the Markstein number and the second one is based on the ratio of consumption speed of strained flames over the laminar unstretched flame speed. This prediction can guide model selection and help manufacturers and experimentalists identify relevant operating points based on desired energy output.